Descriptions

Various types of mass movement features are found in the drainage
basin of the East Fork Coquille River in the southern Oregon Coast
Range. The distribution and forms of mass movement features in the area
are related to geologic factors and the resultant topography.
The Jurassic Otter Point Formation, a melange of low-grade metamorphic
and marine sedimentary rocks, is present in scattered outcrops in
the southwest portion of the study area but is not extensive. The
Tertiary Roseburg Formation consists primarily of bedded siltstone and
is compressed into a series of west to northwest-striking folds. The
overlying Lookingglass, Flournoy, and Tyee Formations consist of rhythmically
bedded sandstone and siltstone units with an east to northeasterly
dip of 5-15°decreasing upward in the stratigraphic section. The
units form cuesta ridges with up to 2000 feet of relief.
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The distribution of mass movements is demonstrably related to the
bedrock geology and the study area topography. Debris avalanches are
more common on the steep slopes underlain by Flournoy Formation and
Tyee Formation sandstones, on the obsequent slope of cuesta ridges,
and on north-facing slopes.
Soil creep occurs throughout the study area and may be the primary
mass movement form in siltstone terrane, though soil creep was
not studied in detail. Slump-earthflows, rockfalls, and rock slumps
also occur in the study area though less extensively than debris
avalanches.
Stratigraphy and bedrock attitude contributed to the pre-historic
occurrence of a major landslide involving Flournoy and Tyee Formation
bedrock. The Sitkum landslide dammed the East Fork Coquille River,
forming a substantial lake which is now filled with sediments. The
form and size of the Sitkum landslide is similar to other landslides
which have dammed drainages in the Coast Range, including Loon Lake,
Triangle Lake, and Drift Creek.
Comparisons with the Loon Lake landslide, which has a known radiocarbon
date, provide estimated dates of 3125 years B.P. for the
Sitkum landslide and 10,300 years for the Triangle Lake landslide.